This invention relates to drive circuits for a power MOSFET switching transistor, and more particularly to a transformer-based isolation drive circuit for such a power MOSFET.
High-power transistors, such as MOSFETs ("Metal Oxide Semiconductor Field Effect Transistors"), are commonly used as switching devices to apply a relatively large electrical current and/or voltage to a load. The load, for example, may be part of a motor or power supply. The MOSFET is a voltage-controlled device having an input gate terminal that controls the bistable output switching states of the device. The bistable output states comprise a conductive or "ON" state, and a non-conductive or "OFF" state. The gate can be controlled by a bistable voltage signal that is of relatively much lower voltage than the switched voltage at the MOSFET output, due in part to the high impedance associated with the gate circuit of the MOSFET. Certain applications of MOSFET drive circuits require DC isolation between the load and the control signals used to drive the MOSFET gate.
One known way of DC isolation involves the use of transformers. The transformer provides the requisite complete DC isolation, along with impedance matching and voltage step-up or step-down ability. However, transformer isolation is inherently limited by the constant volts/seconds property of the transformer. That is, when used to transmit alternating voltage waveforms that have both positive and negative excursions, the core flux of the transformer must be reset after application of positive voltage by applying a negative voltage, and vice versa. The product of the magnitude (volts) and duration (seconds) of each voltage polarity must be equal. Otherwise, residual flux will accumulate in the transformer core. This inherent limitation on transformer performance has typically limited transformer drivers to duty cycles for the control signal of approximately fifty percent. Some examples of prior art transformer isolation circuits, including those which purport to provide for a large duty cycle, are given in U.S. Pat. Nos. 4,423,341, 4,461,966, 4,511,815, 4,967,101, 5,055,722, and 5,304,863.
It is also known to use an optocoupler to electrically isolate the input control signal from the power switching device. In such a scheme, no transformer is utilized, which usually allows for a wide variation in the duty cycle of the input signal. However, an optocoupler scheme requires conversion of the input control signal from electrical to optical energy and back. This leads to inherent circuit complexities and inefficiencies, and degradation of optocoupler performance over time.
Accordingly, it is a primary object of the present invention to provide a drive circuit for a power switching device in that the drive circuit uses a transformer to isolate the low voltage level control signal from the relatively high power switching device.
It is a general object of the present invention to provide a drive circuit that allows a low voltage, bistable-state switching signal to control the bistable switching states of a relatively high powered switching device, such as a MOSFET.
It is another object of the present invention to provide a transformer-based isolation circuit that allows for a wide variation in the duty cycle of the input control signal.
It is still another object of the present invention to provide a transformer-based isolation circuit that has inherent voltage clamping or latching of the voltage control terminal of the power switching device (e.g., the gate terminal of a MOSFET), thereby making the voltage at the output of the MOSFET independent of the saturation of the transformer.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.